4.6 Meteorology Trajectory Error

Computational error was not a significant source of trajectory error. However, spatial and temporal resolution of the meteorological data may be important. We can examine the effect of meteorological data resolution by re-running the forward calculation from 750 m but using the global reanalysis (2.5 deg every 6 h) instead of the NARR (32 km every 3 h).

Open the trajectory setup menu and retrieve traj_fwrd_control.txt. To change the meteorology file, first press the clear button and then Add meteorology files and select captex2_gblr.bin, rename the output file from tdump_fwrd to tdump_gblr, save, and then run the model. Retrieving a forward calculation into the menu that was previously used for a backward calculation changes the run duration from negative to positive but it does not update the radio-button. The negative run duration is the trigger for the backward calculation.

The trajectory should then be displayed and superimposed over the original trajectory (tdump_fwrd). Note how the differences between the two trajectories expand dramatically after 24 hours. The labels on your plot may differ from the one shown here because the plot domain and labels are determined by the first file defined in the list of files to plot.

A trajectory represents the integration of the position vector in space and time through the time- and space-varying wind field. In the atmosphere, the wind direction and speed is a continuous field, but the numerical representation used in these calculations is discreet in space and time. How well the gridded meteorological data represents the real atmospheric conditions can vary from day-to-day and depends upon the location of regions with sharp gradients or cols with respect to the grid points. This is probably the most important source of trajectory error.